The present invention relates generally to the packaging of electronic components. More particularly, the present invention relates to a method of fabricating and using an image sensor package.
Image sensors are well known to those of skill the art. An image sensor included an active area, which was responsive to electromagnetic radiation. To avoid obstructing or distorting the electromagnetic radiation which struck the active area of the image sensor, it was important to avoid contamination, e.g., from dust, of the active area.
Image sensors were fabricated from a silicon wafer. More particularly, a plurality of image sensors were formed in a single silicon wafer. The silicon wafer was singulated, sometimes called cut-up or diced, to separate the image sensors from one another. However, during this wafer singulation, silicon shards were generated. These silicon shards had a tendency to contaminate and scratch the active areas of the image sensors. As a result, image sensors were damaged or destroyed, which undesirably decreased the yield. However, to reduce cost, it is important to have a high yield.
The singulated image sensor was then used to fabricate an image sensor assembly. In this assembly, the image sensor was located within a housing, which supported a window. Radiation passed through the window and struck the active area of the image sensor, which responded to the radiation.
Beaman et al., U.S. Pat. No. 5,821,532, which is herein incorporated by reference in its entirety, teaches an image sensor assembly. In the assembly, an image sensor was mounted to a printed circuit board. After the image sensor was mounted, a housing was mounted around the image sensor and to the print circuit board. This housing provided a hermetic like seal around the image sensor, while at the same time, supported a window above the image sensor.
As the art moves to smaller and lighter weight electronic devices, it becomes increasingly important that the size of the image sensor assembly used within these electronic devices is small. Disadvantageously, a conventional image sensor assembly required a housing to support the window and to hermetically seal the image sensor. However, this housing was relatively bulky and extended upwards from the printed circuit board a significant distance, e.g., 0.100 inches (2.54 mm) to 0.120 inches (3.05 mm) or more. As a result, the image sensor assembly was relatively large.
In addition, mounting this housing at the printed circuit board level was inherently labor intensive and added complexity to the manufacture of the image sensor assembly. In particular, the image sensor was exposed to the ambient environment up until the housing was mounted to the printed circuit board. Since the image sensor was sensitive to dust as well as other environmental factors, it was important to manufacture the image sensor assembly in a controlled environment such as a cleanroom. Otherwise, there was a risk of damaging or destroying the image sensor. Since neither of these alternatives are desirable and both are expensive, the art needs an image sensor assembly which is simple to manufacture so that the cost associated with the image sensor assembly is minimized.
In the event that moisture was trapped inside of the housing, defective operation or failure of the image sensor assembly was observed. More particularly, the moisture had a tendency to condense within the housing and on the interior surface of the window. Even if the housing later dried out, a stain was left on the window. In either event, electromagnetic radiation passing through the window was distorted or obstructed by either moisture condensation or stain, which resulted in defective operation or failure of the image sensor assembly.
For this reason, an important characteristic was the temperature at which condensation formed within the housing of image sensor assembly, i.e., the dew point of the image sensor assembly. In particular, it was important to have a low dew point to insure satisfactory performance of the image sensor assembly over a broad range of temperatures.
In accordance with the present invention, an image sensor package includes an image sensor having an active area, a window, and a window support in contact with the active area and in contact with the window. The window support entirely encloses, and thus protects, the active area of the image sensor.
During use, radiation is directed at the image sensor package. This radiation passes through the window, passes through the window support, and strikes the active area of the image sensor, which responds to the radiation. The window and the window support are transparent to the radiation.
In one embodiment, the refractive index of the window support is similar to the refractive index of the window. In this manner, the sensitivity of the image sensor package is improved compared to the prior art.
Recall that in the prior art, a housing was mounted around the image sensor and to the print circuit board. This housing supported a window above the image sensor. However, located between the window and the image sensor was air. Disadvantageously, air has a relatively low refractive index compared to the window. As those skilled in the art understand, as visible light or other electromagnetic radiation passes from a material having a high refractive index to a material having a low refractive index and vice versa, a significant percentage of the electromagnetic radiation is reflected. Since the electromagnetic radiation had to pass from air, through the window, and back through air to reach the active area of the image sensor in the prior art, a significant percentage of the electromagnetic radiation was reflected. This resulted in an overall loss of sensitivity of prior art image sensor assemblies.
In contrast, the window and the window support of the image sensor package in accordance with the present invention have a similar refractive index. Accordingly, the amount of reflected radiation is reduced compared to the prior art. This improves the sensitivity of the image sensor package compared to prior art image sensor assemblies.
Further, instead of having air between the window and the active area of the image sensor as in the prior art, the window support completely fills the region between the window and the active area. Advantageously, by eliminating the prior art cavity between the active area and the window, the possibility of moisture condensation within the cavity is also eliminated. Accordingly, the image sensor package does not have a dew point.
In contrast, prior art image sensor assemblies had a dew point, i.e., a temperature at which condensation formed within the housing, which enclosed the image sensor and supported the window. Disadvantageously, this limited the temperature range over which the image sensor assembly would satisfactorily perform. Alternatively, the image sensor assembly was fabricated in a low humidity environment to avoid trapping moisture within the housing and was hermetically sealed by the housing to keep out moisture. This added complexity, which increased the cost of the image sensor assembly. Further, in the event that the hermetic seal of the housing failed, the image sensor was damaged or destroyed.
Since the image sensor package in accordance with the present invention does not have a dew point, the image sensor package operates satisfactorily over a broader range of temperatures and, more particularly, at lower temperatures than image sensor assemblies of the prior art. Further, since the image sensor package is formed without a cavity, there is no possibility that moisture will leak into the image sensor package. Accordingly, the reliability of the image sensor package is greater than that of the prior art.
Further, the housing of a prior art image sensor assembly was typically formed of ceramic, which was relatively expensive. Advantageously, the image sensor package in accordance with present invention eliminates the need for a housing of the prior art. Accordingly, the image sensor package is significantly less expensive to manufacture than an image sensor assembly of the prior art.
Further, since the window is attached directly to the image sensor by the window support, the image sensor package can be made relatively thin compared to a prior art image sensor assembly. To illustrate, a 0.039 inch (1.0 mm) or less thickness for the image sensor package is easily achievable.
In contrast, the prior art image sensor housing was relatively bulky and extended upwards from the printed circuit board a significant distance, e.g., 0.100 inches (2.54 mm) to 0.120 inches (3.05 mm) or more. Since the image sensor package in accordance with the present invention can be made relatively thin, the image sensor package is well suited for use with miniature lightweight electronic devices, which require thin and lightweight image sensor assemblies.
Also in accordance with the present invention, a method of forming an image sensor package includes applying a drop to an active area of an image sensor, and pressing a window into the drop to form a window support. By forming the drop with an apex at, or near, a center of the drop, the window initially contacts the apex of drop during pressing. As the window is pressed into the drop, the drop is squeezed by the window downwards towards the active area of the image sensor and outwards from the apex of the drop. Squeezing the drop in this manner avoids bubble formation, i.e., avoid entrapment of air under the window and within the window support. These bubbles would otherwise distort radiation passing through the window support.
In one embodiment, the image sensor is integrally connected to a plurality of image sensors as part of an image sensor substrate. In accordance with this embodiment, drops are applied to the active areas of the image sensors, and windows are pressed into these drops to form window supports on the active areas. The window supports are setup, e.g., cured, gelled, or made tacky.
The image sensor substrate is then singulated to form a plurality of image sensor packages. Of importance, the active areas of the image sensors are protected by the windows during singulation. More particularly, the windows protect the active areas from contamination and scratching during singulation, e.g., from silicon shards. As a result, damage or destruction of the image sensors is avoided. Accordingly, yield is increased and package cost is reduced compared to the prior art.
Further, the windows protect the active areas during subsequent processing of the image sensor package, e.g., during subsequent wire bonding and/or encapsulation and/or molding. More particularly, the windows protect the active areas from dust and contamination. Accordingly, after attachment of the windows to the image sensors, the image sensor packages can be stored or further packaged in any facility with or without a cleanroom.
Recall that in the prior art, the housing, which hermetically sealed the image sensor, was mounted directly to the larger substrate. Thus, to avoid damage or destruction of the image sensor, the image sensor had to be carefully stored and packaged in a cleanroom. Since the prior art requirement of carefully storing and packaging the image sensor in a cleanroom is eliminated, the cost associated with the image sensor package is reduced compared to the prior art.
Further, by forming a plurality of image sensor packages simultaneously, several advantages are realized. One advantage is that it is less labor intensive to handle and process a plurality of image sensor packages simultaneously rather than to handle and process each image sensor package on an individual basis. Another advantage is that usage of materials is more efficient when an array of image sensor packages is fabricated. By reducing labor and using less material, the cost associated with each image sensor package is minimized.
In one embodiment, to form an image sensor assembly, the image sensor package is further packaged. Advantageously, since the active area of the image sensor is protected by the window, the image sensor package can be further packaged using any one of the number of conventional packaging techniques. For example, the image sensor package is put into a flip chip, a leadframe, or a wirebonded image sensor assembly.
Further, the window is formed with a locking feature, which mechanically locks the window to the package body of the image sensor assembly. Since the window is mechanically locked to the package body, the window support can be formed with less structural integrity and adhesion to the window than otherwise would be required if the window support was entirely responsible for supporting the window.
These and other features and advantages of the present invention will be more readily apparent from the detailed description set forth below taken in conjunction with the accompanying drawings.